The present invention relates to bi-level display systems and, in particular, to a method and arrangement for controlling the brightness and contrast of gray-scale, or continuous-tone, images presented by such systems.
At the heart of a bi-level display system is a display panel typically comprising a matrix of individual, closely spaced display cells each of which resides in one of two visual states. That is, each display cell is either completely energized (on) or completely de-energized (off). A plasma panel, for example, is one known type of bi-level display panel. Picture images and other graphic data are readily displayed on a bi-level display panel via selective energization of its cells.
A gray-scale, or continuous-tone, image is characterized by regions of reflected or emitted light of various intensities. To reproduce the image on a display medium, the intensities of these various regions are mapped into, i.e., represented by, correspondingly intense, or bright, regions of the display medium. The greater the intensity of a region of the image, the brighter the spatially corresponding region of the display panel. Unfortunately, an individual cell of a bi-level display panel can represent only one of two extreme intensities, corresponding to the on and off states of the cell. Thus unlike a cathode-ray tube, for example, a bi-level display panel cannot reproduce gray-scale information at each individual display cell location. However, it is known that the above-described mapping can be substantially achieved by representing the average intensity of a region of the image by a region of the display panel having a corresponding average brightness.
One such average-intensity mapping method scans the image to be reproduced in a matrix of picture elements, each corresponding to a different cell of the panel. The intensity of each picture element is quantized into one of a plurality of intensity levels between predetermined upper and lower quantization limits, or levels. Each cell of the display panel is assigned a respective threshold value in accordance with a selected one of several known criteria, as discussed more fully hereinbelow. The intensity of each picture element is compared to the threshold value assigned to the corresponding display cell. If the intensity is greater than the assigned threshold value, the cell is energized. Otherwise it is maintained de-energized.
As indicated above, any of several known criteria, or techniques, can be employed to generate the threshold values to be assigned to the cells of the display panel. One particularly advantageous technique is known as the "ordered dither," or more simply "dither," technique. In a so-called "dithered display system," the cells of the display panel are divided into a plurality of submatrices of predetermined size such as sixteen cells in a four cell-by-four cell arrangement. Each cell within each submatrix is assigned a spatially corresponding threshold value from a predetermined "dither matrix" having (in this example) sixteen threshold values. The dither threshold values are conventionally distributed between the above-mentioned quantization limits with substantially equal spacing between numerically successive threshold values.
A number of other known techniques derive the threshold value to be assigned to each display cell from the characteristics of the scanned image. These techniques are more complex than the dither technique, described above, but may provide improved reproduction of certain types of images, e.g., printed text.
The present invention is directed to a method and arrangement for controlling the brightness and contrast of continuous-tone images which are presented on bi-level display panels via the above-described thresholding. The brightness of an image is determined by the total amount of light emitted or reflected therefrom. The contrast of an image, on the other hand, is determined by the relative difference in brightness between light and dark areas of the image. In a highly contrasted image, for example, the lighter areas are very bright, the darker areas are very dark and little of the image is represented by intermediate, or gray, tones. In a poorly contrasted image, little of the image is very light or very dark and thus there is relatively little difference in brightness between the lightest and darkest areas. The amounts of brightness and contrast which are most pleasing for any particular displayed image are subjective with the individual viewer. Thus it is desirable in many display applications to provide the viewer with a way of adjusting these image characteristics to suit himself.
One obvious approach to controlling the brightness and contrast of a displayed continuous-tone image is to alter the brightness and contrast of the image to be displayed before it is scanned by the display system. This may be accomplished, for example, by varying the intensity and/or frequency content of the light which is directed onto the image. This is usually not practical, however. The required apparatus is bulky, relatively expensive and not easily controlled by the viewer. Moreover, this approach does not permit persons viewing the image on different display panels to individually adjust the displayed image to suit themselves.
Another approach to controlling the brightness and contrast of a displayed continuous-tone image is to appropriately preprocess the scanned image signal in response, for example, to viewer operation of separate brightness and contrast control knobs. The preprocessed signal is then processed and displayed as a continuous-tone image in the manner described above. A principal disadvantage of this preprocessing approach is that it may be difficult, expensive or inconvenient to modify an existing display system to accept the appropriate image signal preprocessing circuitry.